The present invention relates to a direct-injection internal combustion engine.
More specifically, the invention relates to engines comprising a camshaft and at least two cylinders; in each cylinder there moves a piston which is associated with at least two inlet valves, each placed in an induction pipe, and with at least one exhaust valve.
In the direct-injection engines customarily encountered, the air/fuel mixture is prepared in such a way as to encourage a drop in fuel consumption and CO2 emissions. To do that, it is known practice to make shaped piston crowns. However, this may not go far enough in the case of engines of small cylinder capacity in that the average path taken by the fuel injected into the cylinders is very short, which may give rise to misfires through lack of evaporation. It is then necessary to stabilize combustion by mixing the air/fuel mixture around.
To do this, it is known practice to create gas velocity fields inside the combustion chamber by using combinations of correctly shaped pistons and mechanical devices situated upstream of the inlet valves. This then yields two types of turbulent velocity field: one of the velocity fields is said to swirl, that is to say that the gases flow at right angles to the axis of the cylinder, while the other of the velocity fields is said to tumble axially in that the gases move parallel to the axis of said cylinder.
However, these mechanical devices allied with the shape of the piston crown, are expensive and do not make it possible, at all engine speeds, to obtain suitable tumbling and swirling velocity fields. What is more, the dead volumes created in the induction pipes in which the mechanical devices are fitted, may have undesirable gas or gas/fuel mixture reflux effects.
To overcome these drawbacks, it has already been proposed, particularly in DE 198 10 466, for the movement of said valves to be independent of the rotation of the camshaft and controlled by a computer which, cylinder by cylinder, controls the opening and closing times of said at least two inlet valves taking account, in particular, of the engine speed. However, the control means needed, according to DE 198 10 466, are complex and costly.
It is an object of the present invention to alleviate these drawbacks using means which are both simpler and more accurate and have higher fluidic efficiency.
To this end, according to the invention, an internal combustion engine of the aforementioned type known from DE 198 10 466 is characterized in that the induction pipes are shaped to give rise to a field of air velocities inside the cylinder which is parallel to the axis of this cylinder and in that the computer controls the movement of said at least two inlet valves to adapt the shape of the air velocity field inside the cylinder to the engine speed, and to create a velocity field chosen from velocity fields parallel to the axis of the cylinder and ones which swirl around the axis of the cylinder.
Thus, by virtue of these arrangements, it is possible at any moment to control the velocity field inside the cylinder through electronic control of the valves.
In some preferred embodiments of the invention, recourse may possibly be had to one and/or other of the following provisions:
the engine is a controlled-ignition engine;
the shape of the crown of the piston is shaped so that as the piston moves, it causes the air velocity field either to be axial tumbling or to swirl;
the computer controls the speed of travel of each inlet valve;
the computer controls the inlet valves to give rise to a swirling velocity field when the engine is operating at light load and an axial tumbling velocity field when the engine is running at heavy load;
the computer controls the inlet valves so as to obtain a continuous transition between one of the velocity fieldsxe2x80x94axial tumbling and swirling and the other of the velocity fieldsxe2x80x94swirling and axial tumbling; and
the computer controls the opening and closing times of said at least one exhaust valve.